I just completed the crushing three stars quest yesterday and i wanted to do a quick feedback that may be of help. I am a very "normal" player, not very good at headshot or any other "pro" ability so it really is possible to get the three stars crushing trophy. The different elements that I suggest will have been already mentioned but I feel it is interesting to provide them another time.
The different stuff that may help are:
(i) In accessibility option make sure that you have select the ability to press and hold down a button instead of tapping for the QTE when Avery captures you. That way you will not fail a precious attempt at damaging him;
(ii) Also in accessibility, increase to max the sensitivity of movement when you are aiming. That way when you use the M14 custom, you will move your sniping much more quickly. Again very basic stuff but it helped me a lot;
(iii) Easy to say but you NEED to have a solid team especially for stage 7,8,9 and 10. The other stages are ok to play with randoms but the final stages are just nearly impossible if you have not prepared a little bit and shared the overall strategy. I have met two awesome guys to play with randomly. I just send them invites after meeting them the first time and we used the chat to plan for each stage. So even if you start alone it is very easy to meet lots of people randomly who are good and dedicated players and who will accept to team up with you for the long run;
(iv) Be dedicated. Honestly it took us nearly three to four hours to achieve each final stage. You really need to keep practicing, improving the placement and knowledge you have of the map. Do not just give up after three / four fails. Just keep going at it. The progress may be slow but the fact that the waves are always the same helps a lot;
(v) Sad to say but we would not have managed to get the trophy without one use boosters. You need to be prepare to spend a lot of them especially for wave 8, 9 and 10, especially boosters such as team cheap mystical, team cheap heavy weapons and Phoenix;
(vi) You have to get your long guns at level 5. My pistols were only level 4 but level 5 long guns are definitely a must have. Mercifully enough, stage 6 is pretty easy even with randoms, you will discover quickly enough the hanging trick;
(vii) Mines are your BEST friend. Indeed, main advantage with mines is that the enemies and especially the hunters will retreat in front of them giving you more time to nail them;
(viii) For stage 7, just go through stage 6 first to get the cash. It makes the siege for stage 7 much easier (mines and el dorado purchase);
(ix) For stage 10, it is important that the team stays together for the first warlord so when you get him in vulnerable state, you all shoot him. If only one guy shoots it takes way too much time. So just stay together and look at your partner to see if they have an rpg or a grenade launcher in their hand. That will mean they will get the boss in vulnerable state (no need to waste two rpgs for one vulnerable state);
(x) Finally I’ll be honest, I have only been able to complete stage 2, 8 and 10 thanks to glitches. For stage 2 it’s pure luck, I spent nearly 10 hours trying to trigger it with randoms until it finally works. For stage 8 and 10 the glitches are easily triggered. A big thanks to Marinosso for his intel for stage 10 glitch. A bit of warning though glitches for stage 8 and 10 only gives three stars to the host of the game. So you’ll need to complete those stages three times for the whole team to get the stars. First game all players would stay, then second game the player who had already gained the stars would leave the game as soon as the boss waves starts and finally for the last game, we would repeat the same process so the only player missing the stars remain for boss wave. That way we would be sure that we knew who the host was. Video for both glitches are available on the net so I will not dwell on it. Marinosso will also provide you with dedicated intel for the glitch in stage 10 if asked by PM.
Again this was some very general stuff but it really helped us along the way. I played solo with randoms until I got all my level 5 weapons and I had done stage 1 crushing by myself after that it was really team work. Good luck to everyone and a big thank again to Wolf_Town_Spider and Mike_1983 !!